System for continuous growing of monocrystalline silicon

ABSTRACT

An improved system based on the Czochralski process for continuous growth of a single crystal ingot comprises a low aspect ratio, large diameter, and substantially flat crucible, including an optional weir surrounding the crystal. The low aspect ratio crucible substantially eliminates convection currents and reduces oxygen content in a finished single crystal silicon ingot. A separate level controlled silicon pre-melting chamber provides a continuous source of molten silicon to the growth crucible advantageously eliminating the need for vertical travel and a crucible raising system during the crystal pulling process. A plurality of heaters beneath the crucible establish corresponding thermal zones across the melt. Thermal output of the heaters is individually controlled for providing an optimal thermal distribution across the melt and at the crystal/melt interface for improved crystal growth. Multiple crystal pulling chambers are provided for continuous processing and high throughput.

RELATED APPLICATION

This patent application claims the benefit of U.S. patent applicationSer. No. 10/789,638, filed Feb. 27, 2004, now U.S. Pat. No. 7,635,414.

TECHNICAL FIELD

The field of the invention generally relates to growing silicon crystalsby the Czochralski (CZ) technique. In particular, the field of theinvention relates to a system and method for continuous and rapidgrowing of ultra pure, high minority carrier lifetime mono-crystallinesilicon based on improvements to the Czochralski method.

BACKGROUND ART

Referring to FIGS. 1A and 1B, to be useful for the fabrication ofsemiconductor electronic components, silicon must be formed into a large(about 10-30 cm diameter), nearly perfect, single crystal, since grainboundaries and other crystalline defects degrade device performance.Sophisticated techniques are needed to obtain single crystals of suchhigh quality. Referring to FIGS. 1A and 1B, to be useful for thefabrication of semiconductor electronic components, silicon must beformed into a large (about 10-30 cm diameter), nearly perfect, singlecrystal, since grain boundaries and other crystalline defects degradedevice performance. Sophisticated techniques are needed to obtain singlecrystals of such high quality. Referring to FIGS. 1A and 1B, to beuseful for the fabrication of semiconductor electronic components,silicon must be formed into a large (about 10-30 cm diameter), nearlyperfect, single crystal, since grain boundaries and other crystallinedefects degrade device performance. Sophisticated techniques are neededto obtain single crystals of such high quality. These crystals can beformed by either the Czochralski (CZ) technique or the float-zone (FZ)method.

Referring to FIGS. 1A and 1B, in a conventional CZ technique, pieces ofpolysilicon are first melted in a fused silica crucible 100 in an inertatmosphere (typically argon) within a growth chamber 102 and held at atemperature just above 1412 degrees C., the melting point of silicon. Ahigh quality seed crystal 101 with the desired crystalline orientationis then lowered through pull chamber 106 into the melt 122 while beingrotated. The crucible 100 is simultaneously rotated in the oppositedirection to induce mixing in the melt and to minimize temperaturenon-uniformities. A portion of the seed crystal is dissolved in themolten silicon to remove strained outer portions and to expose freshcrystal surfaces. The seed is then slowly raised or pulled from the melt122 by a crystal pulling mechanism 108. As the seed is raised, it coolsand material from the melt adheres to it, thereby forming a largercrystal or ingot 103. Under the carefully controlled conditionsmaintained during growth, the new silicon

A problem in a conventional CZ process arises when a high temperaturecharge of molten silicon 122 is heated within a typical narrow diameter,high width, high aspect ratio crucible 100 by means of heater elementsdisposed around the vertical walls of the crucible. Driving heat thoughthe crucible walls to heat the charge creates stress on the crucible andshortens its useful life. After each growth cycle, the molten siliconremaining in the bottom of the crucible solidifies and expands to suchan extent that it can break the crucible. Thus, in a conventional CZprocess the crucible is generally a single use item.

The silicon must be continuously heated to remain molten in thecrucible. Thus, referring to FIG. 1B, in a conventional high aspectratio, narrow diameter CZ crucible 100 with heaters 118 disposed aroundthe vertical walls of the crucible, the temperature distribution thoughthe melt is characterized by a high thermal gradient and largetemperature difference between the hot walls of the crucible and thecoolest spot at the center of the crystal in the solidification zone atthe melt/crystal interface as shown at 109. Consequently there is asignificant radial temperature gradient and convection velocity gradientacross the solidification zone at the melt/crystal interface and theregion adjacent to the walls are driven to an undesirably hightemperature with attendant excess convection current velocity andthermal perturbations. This condition is sub-optimal for maximized pullrate of high quality defect free crystal. In order to grow more, highquality silicon at a faster rate, a different crucible and heater designis needed that provides a uniform temperature distribution withminimized thermal gradient and convection velocity gradient in thesolidification zone at the crystal/melt interface 107.

Conventional CZ grown silicon differs from ideal monocrystalline siliconbecause it includes imperfections or defects that are undesirable infabricating integrated circuit devices or high conversion efficiencysolar cells. Defects in single crystal silicon form in the crystalgrowth chamber as the crystal cools after solidification. Defectsgenerally are classified as point defects or agglomerates(three-dimensional defects). Point defects are of two general types:vacancy point defects and interstitial point defects.

In a vacancy point defect, a silicon atom is missing from one of itsnormal positions in the silicon crystal lattice. This vacancy gives riseto the point defect.

An interstitial point defect occurs when an atom is found at anon-lattice site (interstitial site) in the silicon crystal. If theconcentration of such point defects reaches a level of criticalsaturation within the single crystal silicon, and if the mobility of thepoint defects is sufficiently high, a reaction, or an agglomerationevent, may occur.

In a conventional CZ process, point defects are generally formed at theinterface between the silicon melt and the solid silicon. Such defectsarise, in part, due to thermal perturbations around the crystalresulting from convection currents and the inability to closely controland or maintain an optimal temperature distribution particularly in thesolidification zone at the crystal/melt interface.

Therefore, what is also needed is an improved heating system withmultiple separate heating zones to aid in controlling crystal formationrates and defect density. Also, such a configuration shouldsubstantially eliminate convection currents and thermal perturbationsthat lead to the formation of point defects. It also would be desirableto minimize the radiant energy that strikes the crystal during growth,allowing for more rapid cooling of the crystal and higher pull rates. Ina conventional CZ process the hottest surface is that part of thecrucible wall not submerged in the melt. A high aspect ratio cruciblebrings this surface in close proximity to the cooling ingot, inhibitingoptimal cooling of the ingot largely through heating by radiation.

Another problem with conventional CZ grown silicon is that it contains asubstantial quantity of oxygen. This is due to the composition andconfiguration of the typical high-aspect ratio, narrow diametercrucible, wherein convection currents scrub the walls of the crucibleand convey impurities into the melt and ultimately to the crystalitself. The convection currents add oxygen to the melt resulting fromthe slow dissolution of fused silica (silicon dioxide) on the

walls of the crucible holding the molten silicon. This introduction ofoxygen into the melt can cause defects in the finished crystal.

In photovoltaic and other applications, high oxygen content in thesilicon adversely affects minority carrier lifetime and greatly degradesperformance and in photovoltaic devices reduces the conversionefficiency.

Thus, what is needed is a crucible design that can minimize theintroduction of oxygen into the melt and provide substantially oxygenfree silicon characterized by high minority carrier lifetime forphotovoltaic and other applications. The use of a special coating ormaterial for a crucible that would make the crucible resistant tobreakdown by molten silicon currently is not feasible since the crucibleis a single use item and is broken by solidification of unused siliconduring the cool down period after each use.

Therefore, what is also needed is a new crucible design that enablesuseful crucible lifetime to be extended over many cycles of operationwithout damage, and thus would make a potentially higher cost inertcrucible surface economically feasible.

Additional problems with a conventional CZ process are the inability tocontrol dopant concentrations across the melt and across the resultingcrystal. For many integrated circuit processes a desired dopant densityis added to the silicon. Such dopant concentration is obtained byincorporating a small carefully controlled quantity of the desireddopant element, such as boron or phosphorus into the melt. For accuratecontrol, a small quantity of heavily doped silicon is usually added tothe undoped melt. The dopant concentration in the pulled crystal ofsilicon is always less than that in the melt because dopant is rejectedfrom the crystal into the melt as the silicon solidifies. Thissegregation causes the dopant concentration in the melt to increaseundesirably as the crystal grows. The seed end of the crystal thereforeis less heavily doped than the tail end.

The segregation effect is also a function of conditions includingtemperature. Thus, a non-uniform temperature distribution through thesolidification zone, crystal/melt interface provides an undesirabledopant concentration gradient and attendant resistivity gradient alongthe crystal radius. Accordingly, what is also needed is a simplifiedcrucible design that minimizes segregation and enables dopantconcentration and resistivity to be substantially uniform throughout thecrystal.

TECHNICAL SOLUTION Summary of the Invention

In order to overcome the foregoing disadvantages of conventional CZprocessing systems, an aspect of the invention provides a system forcontinuous crystal growth comprising a low aspect ratio, large diameter,and substantially flat crucible, including an optional weir surroundingthe crystal. The large diameter, low aspect ratio crucible substantiallyeliminates convection currents and reduces oxygen content in a finishedsingle crystal silicon ingot. Multiple crystal pulling chambers aredisposed with respect to the crucible, such that upon completion ofcrystal growth, a first pulling chamber moves a finished silicon ingotout of the growing zone for cooling, and a successive pulling chambermoves to position a new crystal in the growing zone, thereby eliminatingdown time associated with cooling the crystal and providing continuouscrystal growth with high throughput.

A separate level controlled silicon pre-melting chamber provides acontinuous source of molten silicon to the growth crucible. Thisadvantageously eliminates the need for a crucible raising system andvertical travel of the growth crucible during the crystal pullingprocess, thus greatly simplifying the CZ crystal growing system. It willbe appreciated that eliminating the need for vertical travel of thecrucible and the crucible raising system enables controllable heaterelements to be disposed across the base of the growth crucible inaddition to heater elements disposed around the vertical walls.

The low aspect ratio crucible with annular heater elements disposed onthe base advantageously provides a thermal distribution characterized bya low thermal gradient and small temperature difference between thewalls of the crucible and the coolest spot at the center of the crystalin the solidification zone at the melt/crystal interface. Consequentlythe radial temperature gradient and convection velocity gradient acrossthe solidification zone at the melt/crystal interface is significantlyreduced providing uniform and optimal conditions throughout thesolidification zone at the melt/crystal interface. Further, the regionadjacent to the walls is no longer driven to an excess temperature.Thus, convection currents and thermal perturbations that are a chiefcause of point defects are minimized again, contributing to uniform andoptimal conditions.

Minimized convection currents further reduce the quantity of oxygenintroduced into the melt from scrubbing of the quartz crucible walls,enabling growth of crystals having fewer defects and lower oxygencontent than is possible with a conventional CZ system. The continuousflow of molten silicon from the separate pre melting chamber coupledwith the addition of dopant as needed during ingot growth permitscompensation for segregation and establishes a substantially uniformdopant concentration axially (longitudinally) and radially in the growncrystal.

The advantages of oxygen reduction achieved in the finished crystal,coupled with reduction in other impurities and reduction in dislocationsin the crystal lattice, are especially useful for providing enhancedminority carrier lifetime for improved, high conversion efficiencyphotovoltaic devices.

The low aspect ratio crucible increases the distance between the hotcrucible wall not submerged in the melt and the cooling ingot, reducingheating by radiation and allowing optimal cooling of the ingot.

Multiple crystal pulling chambers are rotatably or otherwisesequentially disposed with respect to the crucible, such that uponcompletion of a first single crystal silicon ingot, the first pullingchamber moves the finished silicon ingot out of the growing zone forcooling, and a successive pulling chamber moves to position a newcrystal in the growing zone. The sequential positioning of pullingchambers at the growing zone completes the continuous process,eliminates down time associated with cooling each crystal and changingto a new crucible, and thus provides a system for continuous crystalgrowth with high throughput. Since in a continuous process the crucibleand melt are not cooling and reheating to melt temperature, considerableenergy savings result. Also, the atmosphere around the heaters andcrucible are not opened to ambient atmosphere, thus fewer impurities andcontaminants are introduced into the crystal pulling chamber.

Impurities introduced into the melt, e.g. from scrubbing action ofconvection currents within the crucible, are dependant upon the averagedwell time of silicon in the crucible and the surface area of contactbetween the melt and the crucible, especially the surface area of thevertical walls of the crucible. In an aspect of the invention, the dwelltime is minimized through the improved cycle time and the average areaof contact is minimized through the optimized low aspect ratio crucibledesign. It will be appreciated that these same considerations will applyto the pre-melter whose sizing and therefore dwell time and surface areaof melt contact are optimized.

Another aspect of the invention provides enhanced temperature controlthat maintains a temperature in the melt of approximately 1420° C., suchthat the temperature has an optimized thermal distribution between themelt and crystal for accelerated crystal growth. To achieve such anoptimized thermal distribution at the critical interface between thecrystal and the melt, a plurality of annular, independently monitoredheating elements are disposed in a radial pattern across the base of thegrowth crucible, as close to walls and to the bottom of the growthcrucible as possible. The heating elements are separately controlledwith active feedback to enable rapid thermal response and distribute thethermal workload to achieve an optimized thermal distribution across theinterface between the crystal and the melt and between the crystal andwalls of the crucible. This also contributes to improved crucible life,as well as reduction of oxygen and other impurities, in the finishedsingle crystal silicon.

In contrast to a conventional CZ system, the growth crucible is nolonger a single use item, but rather can be reused over multiple crystalgrowing cycles. Accordingly, growth crucible surfaces in contact withmolten silicon are provided with a coating of alpha or beta siliconcarbide, or similar ceramic coating that is inert with respect to moltensilicon and further safeguards against introduction of oxygen into themelt. It will be appreciated that for similar reasons the pre-melter canbe fabricated from these advantageous materials.

The foregoing aspects of the invention advantageously prevent theintroduction of oxygen into the melt and provide a crucible that iscapable of being used for about 10 crystal growing runs or more, whileachieving a single crystal silicon product of substantially float zonequality with enhanced minority carrier lifetime. The oxygen reductionachieved in the finished silicon crystal is especially useful forproviding enhanced minority carrier lifetime for applications such asimproved conversion efficiency photovoltaic devices.

Advantageous Effects DESCRIPTION OF DRAWINGS Brief Description of theDrawings

The drawings are heuristic for clarity. The foregoing and otherfeatures, aspects and advantages of the invention will become betterunderstood with regard to the following descriptions, appended claimsand accompanying drawings in which:

FIG. 1A is a schematic side view of a conventional CZ system.

FIG. 1B is a schematic side view of a conventional CZ system showing theundesirable temperature gradient through the melt.

FIG. 2 is a schematic side view of a system for growing enhanced puritysingle silicon crystal in accordance with an aspect of the presentinvention.

FIG. 3A is a schematic side view of a crystal growing system showing apre-melter and individually controlled heating elements in accordancewith an aspect of the present invention.

FIG. 3B is simplified top view of the crystal growing system as shown inFIG. 3A in accordance with an aspect of the present invention.

FIG. 3C is a schematic side view of the wide diameter, low aspect ratiocrucible showing improved thermal gradient through the melt inaccordance with an aspect of the present invention.

FIG. 4 is a side view of a pre melter for a crystal growing system inaccordance with an aspect of the present invention.

FIG. 5 is a schematic diagram showing a system for heater control toestablish an optimized thermal distribution across the crucible inaccordance with an aspect of the present invention.

FIG. 6 is a schematic diagram showing a system for level control in thecrucible accordance with an aspect of the present invention.

BEST MODE Detailed Description

Referring to FIGS. 1A and 1B, in a conventional CZ system pieces ofpolysilicon are melted in a fused silica crucible 100 in an inertatmosphere in growth chamber 102. The atmosphere in the chamber 102 istypically argon and is controlled by isolation valve 104 in accordancewith techniques that are well known. The silicon is held in crucible 100at a temperature just above 1412 degrees C., the melting point ofsilicon. A high quality seed crystal with the desired crystallineorientation is lowered in the crystal pull chamber 106 to contact themelt at crystal melt interface 107 in the crucible 100 while beingrotated. Crucible 100 is simultaneously rotated in the oppositedirection to induce mixing in the melt and to attempt to minimizetemperature non-uniformities. A portion of the seed crystal is dissolvedin the molten silicon to remove strained outer portions and to exposefresh crystal surfaces.

The seed is then slowly raised or pulled from the melt in crystal pullchamber 106 by conventional pull mechanism 108. As the seed is raised,it cools and material from the melt adheres to it, thereby forming alarger crystal or ingot 103. The main body of the crystal or ingot 103is grown by controlling the pull rate and the melt temperature whilecompensating for the decreasing melt level in the crucible. That is, asthe crystal grows, the molten silicon in the crucible 100 is depleted.In order to compensate for the changing level of melt in the crucible100 with respect to the heaters disposed around the vertical walls ofthe crucible, the crucible 100 must be elevated vertically in carefullycontrolled fashion from a starting crystal growth position 112 to afinal or end position 114.

Complex mechanisms must be provided to coordinate the vertical travel ofthe crucible with the pulling of the crystal. The vertical travel of thecrucible must be precisely coordinated with the pulling of the crystalin a vertical direction, such that a constant interface between thecrystal and the melt is carefully maintained and the interface betweenthe crystal and the melt is positioned correctly with respect to theheaters.

The diameter of the crystal is controlled by decreasing or increasingits pull rate and/or the melt temperature until the desired or targetdiameter is reached. The initial pull rate is generally relativelyrapid. The pulling continues until the melt is nearly exhausted. It isvery expensive to design equipment that provides precise coordinatedvertical travel of the crucible with the crystal pulling mechanism.

FIGS. 1A and 1B show additional disadvantages associated with aconventional CZ crucible and heater arrangement. A conventional CZcrucible 100 is characterized by a narrow diameter, high aspect ratio.The high aspect ratio is necessary in order to hold all of the moltensilicon for the growing crystal as there is typically no means forreplenishment of the melt. Instead, the crucible 100 must travel in avertical direction (from start position 112 to end position 114) incoordination with the pulling of the crystal as silicon in the crucibleis depleted.

Heater elements 118 are provided around the circumference of thecrucible 100 and produce a temperature distribution in the melt whichdisadvantageously maximizes the thermal gradient (DT) between theheater, the walls of the crucible, the melt and the crystal suspended inthe center of the melt and can cause the walls of the crucible to betaken to excess temperatures. This disadvantageously slows crystalgrowth.

In addition, convection currents are generated in the narrow diameter,high aspect ratio crucible 100. Convection currents adversely affect thepurity of the single crystal silicon. A conventional CZ crucible 100 iscomprised of a material such a fused silica. Molten silicon breaks downthe walls of a conventional fused silica crucible into silicon andoxygen. Convection currents scrub the walls of the crucible and conveyoxygen and other impurities into the melt. This adversely affects thepurity and defect structures in the growing crystal. Convection currentsalso create adverse thermal perturbations around the growing crystalthat may induce defects into the crystal.

Upon completion of crystal growth, residual molten silicon that is notremoved from the crucible 100 greatly expands upon solidification andbreaks the crucible. The typical CZ crucible 100 is therefore a singleuse disposable item that is discarded after each silicon ingot is grown.

Wide Diameter, Low-Aspect Ratio Crucible

Referring to FIG. 2, a crystal growing system according to an aspect ofthe present invention provides a fixed, wide diameter, low-aspect ratiocrucible 200 provided in growth chamber 202, which is in turn providedwith a base 201. A conventional isolation valve 204 provides a vacuum orotherwise controls the atmosphere in growth chamber 202 and multiplecrystal pull chambers 210 a and 210 b in a well-known manner. The widediameter, low aspect ratio configuration of the crucible 200 is providedwith a means for minimizing radiant heat, such as a heat shield 205, forminimizing radiant energy that strikes the crystal or ingot 203 duringgrowth. Heat shield 205 is a planar section of silicon carbide,graphite, or other high temperature material supportably mounted on thewalls of the growth chamber 202 having an interior opening sized toaccommodate the silicon ingot 203. An annular region 211 adjacent theopening is deflected downward toward the melt to decrease heat flowalong the ingot 203 and for minimizing thermal shock when the ingot isremoved from the melt 222.

Referring to FIG. 3A, an optional weir 220 is disposed in the melt 222between the crystal/melt interface and outlet port 228 of siliconpre-melter 208. The weir 220 rests on bottom of the crucible 200 oralternatively can be supported by support means such as any convenientsupporting structure comprised of an inert material provided on theinner walls of the crucible. The top of the weir 220 extends above thesurface of the melt 222. The purpose of the weir is to enable moltensilicon from the pre-melter 208 to be distributed into the melt withoutthe formation of ripples in the melt or thermal perturbations that woulddisturb the temperature distribution in the melt and adversely affectthe growing silicon crystal 224. The weir 220 is characterized by lowheight in relation to its diameter and is generally cylindrical in shapewith apertures provided in that portion of the weir extending beneaththe surface of the melt to enable a desired thermal distribution in themelt.

The wide diameter, low aspect ratio growth crucible 200 also prevents orgreatly reduces formation of convection currents in the melt andattendant scrubbing action upon the crucible, further reducing theintroduction of oxygen. In a preferred embodiment, the low aspect ratio(diameter with respect to height) of the crucible is in a range of 4:1to 10:1 and preferably about 8:1. In contrast, conventional crucibleshave aspect ratios on the order of about 1:1-1:4.

Furthermore, annular heating elements are disposed in a radial patternon or as close as possible to the base of the crucible, which was notpossible in a conventional CZ grower due to the need for a liftmechanism. In addition to the heaters disposed around the circumferenceof the crucible, the annular heaters provide corresponding heating zonesin the melt. This results in an optimal thermal distribution that issubstantially horizontal through the melt. It also provides an optimaltemperature distribution, particularly at the critical interface betweenthe crystal and the melt. The improved temperature control providesaccelerated crystal growth beyond what previously has been possible.

Melting of Granular Poly-Silicon in a Low Aspect Ratio Crucible

Conventional crucibles have high aspect ratios and lift mechanisms suchthat the level of the melt in the crucible, with respect to the heaterson the sides of the crucible, can be kept constant during crystalgrowth. Typically there are no heaters under the base of the crucible.

When such crucibles are charged with poly-silicon material thetemperature distribution during the melt down process is highlynon-uniform. The temperature is highest closest to the walls of thecrucible where it is closest to the heaters and also cooler at the topand bottom of the melting material than at the center. The problem isgreatly exacerbated when the poly-silicon material is in granular formwith small particles (<1 mm diameter), large surface area and minimalpoints of contact between the granules, air being an excellentinsulator. Heat flow between the granules tends to fuse them together attheir points of contact. Additional heat flow is by radiation, verylittle is by convection at this stage. The granules closest to theheaters melt first and those at the edges and center slump down to thebottom of the crucible leaving a bridge of fused granules across the topsurface and an air void beneath the bridge. Liquid silicon of courseoccupies much less volume than the granules. The combined effect is toretard the melting process and care must be taken not to drive melttemperatures close to the heaters to excess levels or contaminationlevels will increase. There are techniques to lessen the problem, butthey are exacting and time consuming e.g. raising the crucible un stagesup through the heaters such that the top of the mass of granules meltsfirst, again being careful not to drive too much heat from the sides.

Referring to FIG. 3B, according to an aspect of the invention, a lowaspect ratio crucible 200, has a plurality of annular heaters 218disposed in a radial pattern beneath its base in addition to sideheaters 219, to provide a much more uniform temperature distributionbecause of a) the lower depth of granules in the crucible and b) annularbase heaters 218 applying heat in a more controlled distribution acrossthe entire base surface area of the crucible. The annular base heaters218 are preferably planar resistive heating elements that areindividually controlled as described with respect to FIG. 5. Eachheating element 218 generates a corresponding heating zone in the meltto provide an optimal temperature distribution through the melt. Thehigher surface area of contact between the granules and the heated wallsof the low aspect ratio crucible, compared to the high aspect ratiocrucible, drives more heat into the granules. Thus, the entire mass ofgranules melts much more uniformly and at a much more rapid rate withoutthe attendant contamination associated with excess temperatures close tothe heaters.

Referring to FIG. 3C, a low aspect ratio crucible 200 with annular baseheaters 218 develops a thermal gradient 223 characterized by asubstantially horizontal distribution through melt 222. In a continuousprocess using a low aspect ratio crucible, compared to a batch process,it is easier to transfer the heat to melt the initial charge because of:a) a smaller amount of poly-silicon ‘charge’ melted at start-up and b)the pre-melter 208 providing liquid silicon to wet the granules provideslarger areas of thermal contact between granules and accelerating themelting process. Note: the pre-melter also is designed to provideuniform heating around the granules rather than heating from one side tofurther enhance the melting process.

Pre Melter

Referring to FIGS. 3A, 4 and 5, the pre-melter 208 comprises a separatecontainment vessel for melting a quantity of solid feedstock materialand for providing a constant source of molten material to growthcrucible 200 for growing crystals. In the case of single crystalsilicon, a source 209 of solid silicon feedstock such as silicon chips,chunks, granules or rods is provided through flow control device 212 tothe pre melter 208 at a rate sufficient to replenish the growthcrucible.

The pre melter can be separately situated apart from the growth chamberas shown in FIGS. 5 and 6. In a preferred embodiment, the pre melter 208comprises a separate containment means provided within the growthchamber 202 for melting a quantity of feedstock and providing it to thesurface of the melt 222. This advantageously places the pre melter 208in the controlled atmosphere of the growth crucible 222 and minimizesthe distance that the melt from the pre-melter needs to travel needs totravel to reach the crucible.

Referring to FIG. 4, the pre melter comprises a melting chamber 400. Thepre melter is comprised of a quartz material capable of withstandingtemperatures up to about 1590 degrees C. Fused silicon carbide, siliconnitride bonded silicon carbide or similar material also may be used forthe pre melter. One or more resistive heaters 402 are suitably disposedbeneath or adjacent the melting chamber for melting a quantity of solidcrystalline feedstock. An optional thermal conductor 404 may bedisposedbetween the heaters 402 and melting chamber 400. Thermal conductor 404also is an electrical insulator. Thermal conductor 404 diffuses thermalflux from the heaters and reduces the maximum temperature seen by thequartz walls of the pre melter. Thermal conductor 404 also providesmechanical support for the quartz melting chamber at temperatures beyondabout 1590 degrees C. Heaters optionally also may be disposed around themelting chamber.

A source of dopant and solid silicon or crystalline feedstock 209 isprovided through a flow controller 312 to an inlet 408 at or abovesilicon level 410 in a first section of the melting chamber 400. A weir414 defines a first portion or section 416 of the melting chamberincluding inlet 408, and also defines a second section 418, including anoutlet to the crucible. The separate first section 416 of the meltingchamber 400, is provided with an inlet 408 for receiving the solidcrystalline feedstock. Inlet 408 also provides a means for receiving apredetermined quantity of solid dopant material either directly orthrough the feedstock and dopant source 209. As a non-limiting example,dopant material can be a dice of heavily doped wafer on the order of0.125×0.125×0.25 inches at a rate of up to 10 dices per ingot.Subsequent ingots in an ingot stream will require less dice. The amountof dopant dice required is a function of the amount of dopant taken upin the crystal as it is grown. That is, the dice simply top up thatdopant which is taken up from the melt into the crystal. Adding dopantin the pre melter avoids thermal perturbations and non-uniformtemperature distribution that otherwise would result from adding solidchunks of dopant into the melt. Such temperature distribution problemswould arise from the latent heat of fusion and thermal capacity(mass×specific heat×D T) to bring the dopant material up to melttemperature. Note that such thermal perturbation problems are much thesame as when adding solid silicon feedstock directly into the melt,although very much reduced. Due to the optimized thermal gradient acrossthe melt and the control of thermal zones in the melt by respectiveindividually controlled heating elements, a uniform thermal distributioncan be maintained across the radius of the growing crystal. Thus, theaddition of dopant material at inlet 408 can provide substantiallyuniform resistivity or conductivity axially (longitudinally) andradially in the finished ingot. First weir 414 is provided with aflow-controlling outlet 420 at the bottom of the first section 416.

Molten silicon enters the second portion 418 of melting chamber 400 fromthe bottom of the first section 416 through outlet 420. The moltensilicon then rises to the level 410 in the first section. Due to thefact that solid granules or unmelted pieces of silicon float, it iscritical to provide a weir 414 in the pre melter to ensure that onlymolten silicon or crystalline feedstock circulates to the bottom of thefirst pre melter section 416 by means of the flow controlling outlet 420of the weir and then fills the second section 418 from the bottomupward. The molten silicon or molten crystalline feedstock that entersthe crucible melt from the outlet 424 is thus taken from the bottom ofthe pre melter. This arrangement advantageously ensures that unmelted,solid material, floating in molten silicon by virtue of its densitylower than molten silicon, does not pass directly through to secondsection 418 of melting chamber 400 and on to the growth crucible.

An outlet tube 424 also acts as a second weir and controls the meltlevel in melting chamber 400 of the pre-melter. Outlet tube 424comprises a tube having an inlet for receiving molten crystallinefeedstock from the second section 418 of melting chamber 400 and anoutlet at its distal end that provides a substantially constant sourcefor replenishment of molten crystalline feedstock to the melt in thecrucible. Outlet tube 424 induces flow of molten crystalline feedstockalong the interior thereof and into the melt in the growth crucible.

Outlet tube 424 is characterized by an inner diameter of a sufficientsize, approximately 1 cm, to overcome surface tension of the moltenfeedstock (surface tension of molten silicon is approximately 30 timesgreater than water). Surface tension tends to stop or limit flow throughthe outlet tube for a given head of molten crystalline feedstock in thepre melter. Thus, the diameter of the tube is optimized to overcomesurface tension, while at the same time minimizing the splash effectthat would cause excessive perturbation in the melt in the crucible. Thedistal end of the outlet tube is positioned at a point above the levelof melt in the crucible, at a height chosen to minimize perturbationswhen discharging molten feedstock and dopant into the melt forcontinuous replenishment of the crucible. The design of the outlet tubethus further maintains static thermal conditions at the crystal meltinterface that result in substantially uniform axial (longitudinal) andradial resistivity or conductivity in the finished ingot.

In this manner pre-melter 208 provides a substantially constant sourceof molten silicon to growth crucible 200, replenishing the silicon thatis being taken up by the growing crystal. This enables the melt ingrowth crucible 200 to be maintained at a constant level with respect toa growing crystal, without the need for vertical travel of the crucibleand also enables the level of the melt in the crucible to be increasedor decreased as required. This advantageously eliminates the complexmechanisms in a conventional CZ system necessary for coordinatingvertical travel of the crucible with the pulling of the crystal. Suchreplenishment by the pre melter also enables heaters to be positioned onthe base of the crucible. This aspect of the invention greatlysimplifies the apparatus needed for growing single crystal silicon andultimately enables accelerated production of single crystal silicon atlower cost.

The substantially continuous addition of melted silicon by the use ofpre melter 208 eliminates the lost time involved and energy wasted inshutting the furnace or heating elements off to recharge the crucible200 and remelt the silicon. The use of a substantially continuous sourceof molten silicon feedstock to replenish the melt minimizes the time themelt is in contact with the crucible, thus further limiting oxygenabsorption into the melt. Since the raw silicon is melted within thepre-melter in very small quantities and immediately flows into thegrowth crucible, dwell time and surface area of contact are likewiseminimized. Furthermore, there is no need to open the growth chamber toambient atmosphere in order to replace the crucible and provide a newsilicon charge, a process introducing new contamination into the growthchamber.

Another advantage of silicon pre-melter 208 is that the axial resistanceof the crystal can be better controlled as dopant can be added duringrecharging. This advantageously eliminates the axial resistivitygradient exhibited in crystals grown by the conventional CZ process. Theeffects of segregation in the melt and resulting non-uniform dopantprofiles in the crystal are substantially eliminated. Yet anotheradvantage of using a separate silicon pre-melter 208 communicating withgrowth crucible 200 is that eliminating the high temperature initialmelting of a silicon charge minimizes the stress on the growth crucibleand lowers the precipitation of oxygen into the melt.

It will be appreciated that the pre-melter can be made from, or coatedwith, an inert material such as sintered silicon carbide or likeceramic, or with other materials characterized by an inert characterwith respect to molten silicon such as tantalum, niobium, or oxides andcompounds thereof, to reduce oxygen and other impurities in the melt, asis the case for the crucible.

The silicon pre-melter 208 in combination with the low aspect ratio,non-reactive, sintered silicon carbide crucible 200 and controllabledopant feed during crystal growth substantially eliminates segregation,high impurity levels and oxygen precipitation that cause defectstructures and sites of minority carrier recombination. This aspect ofthe invention is especially useful in providing higher minority carrierlifetime silicon for high conversion efficiency solar cells.

Multiple Crystal Pulling Chamber

Referring again to FIGS. 2 and 3A, multiple pull chambers 210 a, 210 bare provided on a rotating cylinder 212 that is in turn supported by aspindle 214. It will be appreciated that multiple pull chambers 210 a,210 b also can be arranged in a moveable, linear supporting member forconsecutive positioning of a seed crystal in each successive pullingchamber into the growth zone in crucible 200 within growth chamber 202.Multiple pull chambers 210 a, 210 b thus are rotatably or sequentiallydisposed with respect to the growth chamber 202. Upon completion of afirst single crystal silicon ingot, the first pulling chamber 210 amoves the finished silicon ingot out of the growing zone in crucible 200and out of growth chamber 202 for cooling, and a successive pullingchamber 210 b moves to position a new crystal into growth chamber 202and to the growing zone at a crystal/melt interface in crucible 200. Theisolation valve 206 closes to control the atmosphere in growth chamber202 and the associated pull chamber, and a new crystal is grown.

The sequential positioning of pulling chambers 210 a, 210 b at thegrowing zone in crucible 200, is the final step in the continuousprocess, minimizing dwell time of silicon in the growth crucible,eliminating down time associated with cooling each crystal, changing toa new crucible, recharging the crucible, evacuating the growth chamberand reheating the charge to melt temperature; thus providing anaccelerated, continuous crystal growth system with high throughput.Also, such a continuous process eliminates the single use nature of thegrowth crucible and enables the growth crucible to be used for multiple(10 or more) crystal growing cycles

Composition of Crucible

Referring again to FIGS. 2, 3A, 3B and 3C, another aspect of theinvention provides a low aspect ratio, wide diameter crucible 200comprised of a material that is inert to molten silicon such as alpha orbeta sintered silicon carbide, tantalum nitride, or similar ceramic thatcontains no silica. Alternatively, the interior silicon containingsurface of the crucible 200 may be provided with a coating of such aninert material in accordance with techniques that are well known. Such adesirable inert material is comprised of a mixture of silicon carbidegrains and a sintering aid that is pressed and sintered. Unlikereaction-bonded carbide, there is no free silicon present. Such directsintered materials have no metal phase and are therefore resistant tochemical attack. Alpha silicon carbide refers to a hexagonal structureand beta to a cubic structure.

Such sintered silicon carbide materials are available from CARBORUNDUM

Corp., designated SA-80; from GENERAL ELECTRIC, designated as Sintride,and from KYOCERA, designated as SC-201.

A chemically inert growth crucible 200 comprised of the foregoingsintered silicon carbide materials is unknown in a conventional CZgrowing process, because the conventional crucible is a single use,disposable item, and there is no motivation to provide a sinteredsilicon carbide or ceramic crucible or such a coating on the cruciblesurface.

A conventional CZ growing process does not contemplate the use of acrucible material, such as sintered silicon carbide, to substantiallyeliminate the introduction of oxygen into the melt. In a conventional CZsystem, a growth crucible is typically discarded after one or two growthcycles. Thus, a coating of silicon carbide or a crucible made from ahigher cost material would add significantly to the cost of aconventional CZ system. It will be appreciated that these materials canalso be used advantageously in the pre-melter for similar reasons.

Further, oxygen precipitates originating from the growth crucible wallspreviously were not recognized as a serious problem and even could bebeneficial in integrated circuit and other electronic devices. Oxygenprecipitates form sites on which other impurities tend to accumulate.Such oxygen precipitates can be positioned in a predetermined mannerremotely from an active device region in a finished IC wafer. Oxygenprecipitates then function as gettering sites that attract unwantedimpurities away from electrically active regions, thereby improvingdevice performance.

However, in accordance with an aspect of the invention, oxygenprecipitates and associated defects are recognized as a problem forminority charge carrier lifetime in silicon that is to be used forspecialized applications such as solar cells. In a solar cell, if someof the generated carriers in a photovoltaic cell recombine at defects,impurities, or sites of oxygen

precipitates in the silicon, before reaching the electrical contacts,output current is diminished. Across multiple solar cells, such defectscan seriously decrease output current.

Heater and Melt Temperature Control

FIGS. 3B, 3C and 5 show a heater and melt temperature control systemthat provides closed loop control of temperature characterized byoptimal temperature distribution across the melt and uniform optimalthermal conditions in the solidification zone at the melt/crystalinterface 207 aiding in controlling crystal formation rates and furtherminimizing defect density.

Referring to FIGS. 3B and 5, a plurality of annular resistive heatingelements 218 are disposed in a radial pattern beneath the low aspectratio crucible 200. Additional resistive heating elements 219 aredisposed around the circumference of the outer wall of crucible 200.Annular heating elements 218 and sidewall heating elements 219 areindividually controlled by heater control 240 to generate separateheating zones for providing an optimal temperature distribution acrossthe melt. Heater control 240 includes a microprocessor controller forcontrolling and monitoring the activation time, power consumption andconsequently thermal output of each heater element in response tosignals from the sensors 234.

A desired thermal output can be maintained for each separatelycontrollable resistive heating element to thereby achieve optimaltemperature distribution across the melt and across the radius of thegrowing crystal. The desired thermal output and resulting temperaturedistribution are derived by measuring the power consumption of each ofthe individually controllable resistive heating elements through themicroprocessor of heater control 240. Power consumption of each heatercorresponds to the thermal output needed to achieve the optimaltemperature distribution. Heater control 240 applies power to eachheater element in accordance with the monitored power consumption toachieve a repeatable state such that the corresponding thermal zonesdrive heat uniformly into the melt. It is to be understood that theadvantageous heater arrangement and controllable thermal zones also canbe used to drive heat uniformly into a molten material that includessolid granules. The heater arrangement also can be applied to uniformlyheat and melt solids, such as granules, a combination of granules andchips, as well as chips or chunks of solid crystalline feedstock. Withthe optimal arrangement of independently controlled heaters underneaththe low aspect ratio crucible, the thermal path into the charge acrossthe contact points between the pieces of solid insulated material isminimized. This is especially important when small chips or granules areused, as these have many more points of contact between adjacent chipsor granules for a given mass or material, thus reducing or limiting heatflow.

This overcomes problems in conventional CZ systems wherein solidmaterial, especially small chips and granules, closest to the heatersmelt first and those at the edges and center slump down to the bottom ofthe crucible leaving a bridge of fused granules across the top surfaceand an air void beneath the bridge.

Thus, a series of thermal zones representative of an optimal thermaldistribution are established across the melt. Each thermal zonecorresponds to the thermal output of a separately controlled resistiveheater element 218. A temperature sensor 234 comprising one or moreoptical pyrometers takes a temperature reading of each separate thermalzone across the melt, each zone controlled by a corresponding heaterelement. A single pyrometer also may scan separate zones, providing anoutput signal on lead 236 representative of the temperature of eachzone. The temperature sensor 234 also may include a thermocouple forsensing temperature of the outer heating elements 219 disposed aroundthe circumference of the crucible 200.

In accordance with standard closed loop load regulation techniques, thetemperature sensor 234 sends a signal on line 236 representative of thetemperature of each respective thermal zone to heater control unit 240.Heater control unit 240 sends a corresponding activation signal to eachheating element to maintain that heating element in a predeterminedrange. After achieving the desired control set point, heater and melttemperature can be maintained in a narrow range. It will be appreciatedthat individual control of resistive heating elements 218 provides anoptimized thermal distribution between the walls of the crucible and thecrystal. The rate of pull (rate of growth of the crystal) is controlledby the temperature distribution at the interface between the crystal andthe melt. Accordingly, this aspect of the invention provides anoptimized temperature distribution to be maintained substantiallyhorizontally across the melt and particularly at the crystal meltinterface with greater control than was previously possible. It will beappreciated that this optimized temperature distribution is achieved bythe unique design of the wide aspect ratio crucible in combination withthe individually controlled heating elements placed beneath and aroundthe crucible coupled with the lower depth of melt.

Referring again to FIG. 4, since introduction of solid silicon feedstockinto the melt can cause temperature perturbations, improved control ofthe melt is achieved by silicon pre-melter 208 wherein the predominanceof all melting of silicon is done outside the crucible. A source ofsolid silicon feedstock 209 comprises silicon feedstock in a variety offorms such as crushed silicon, chips, chunks, granules from a fluidizedbed, silicon rods, or the like.

As shown in FIG. 5, the rate of addition of solid silicon from source209 to pre-melter 208 may be controlled by melt level sensor 230.Silicon pre-melter 208 is provided with an outlet 422 that dischargesmolten silicon into the melt 222 and behind the optional weir 220 (FIG.3B) without introducing perturbation in the melt. Melt level sensor 230provides improved closed loop control over molten silicon released intothe melt. Level sensor 230 comprises, for example, a 5 mW, 670 nmvisible, Class IIIa laser and photo detector system. A meniscus sensor232 is used to monitor the diameter of the crystal as it grows inconventional optical pattern recognition techniques. Crystal growthtakes place at the meniscus interface 207 between the crystal 244 andthe melt 222 and the pull rate is adjusted to give the desired ingotdiameter

In accordance with an aspect of the invention, a meniscus sensor 232comprising a camera or photo detector system is aimed at the meniscusinterface between the crystal 244 and melt 222. The meniscus sensor 232continuously monitors the position and size of the meniscus interfaceand provides corresponding output signals to the level sensor. In likemanner, the level sensor carefully monitors the melt level. In this way,a predetermined crystal growth rate and diameter can be closelycontrolled with active feedback. If more molten silicon is needed toachieve a predetermined crystal size, the level sensor provides outputsignals to the silicon source control 209 to additional solid silicon tothe pre-melter. The pre-melter 208 in turn releases molten silicon intothe melt.

Level Control

Referring to FIG. 6, in a preferred embodiment, improved level controlof the melt 222 in the growth crucible 200 and the rate of discharge ofmolten silicon feedstock from pre-melter 208 into the melt 222 isachieved by an active feedback system for sensing the weight of thesilicon melt in the growth crucible and adjusting the amount of siliconfeedstock provided to the pre melter, and the amount or rate of moltensilicon discharged from the pre melter 208 into the melt.

A sensitive means for determining the weight of the growth crucible bothempty and with a desired level of melt is provided by a weight sensor300. A suitable weight sensor 300 comprises one or more strain gaugebased load cells. Each load cell is a transducer that converts a load orweight acting on it into an electrical signal representative of thatload. The weight of silicon melt in the crucible 200 produces adeflection of a mechanical beam or arm 304 that is in contact with thecrucible 200. This in turn produces an electrical resistance changeproportional to the load. The load cell or weight sensor 300 thenproduces an output signal representative of the weight of the melt 222to microprocessor based level controller 306 over a communication link308. The communication link 308 can be can be an electrical cable or afiber optic, infra red or wireless link to provide stable hightemperature operation.

In response to signals from weight sensor 300, level controller 306produces output signals over a communication link 310 to actuate adispenser or flow controller 312 that controls the release of apredetermined amount of solid silicon feedstock 209 into the pre melter208. Level controller 306 comprises a microprocessor for determiningoutput of the pre melter based on a desired depth D of melt in thegrowth crucible. According to an aspect of the invention, this isdetermined by the following relationship:D=(W−Wt)/(π*R exponent 2*r);

where W is the total weight of the crucible 200 containing melt 222; Wtis the weight of the crucible 200 measured empty; R is the internaldiameter of the crucible; and r is the density of liquid silicon.

In this manner, it is possible to control the level of the silicon inthe pre melter and the level of the melt 222.

It will be appreciated that the foregoing system provides an optimaloutput capacity of the pre melter and enables a closely controlled,optimized replenishment of the pre melter and growth crucible. Thisadvantageously accelerates throughput by allowing crystal growth to berun with a much lower charge of melt than in a conventional CZ processand contributes to the reduced dwell time of the silicon in the crucibleand attendant reduction in impurities. This further enables a newcrystal to be started more quickly after emptying the crucible through acombination of crystal growth and truncating flow from the pre melter.

The foregoing features of the present invention provide a single crystalsilicon growth process that minimizes the precipitation of oxygen intothe melt and minimizes or eliminates impurities and segregation in themelt. Because those factors minimize impurity levels and defectstructures that give rise to carrier recombination sites, the process ofthe invention directly achieves enhanced minority carrier lifetime insilicon. Such silicon with enhanced minority carrier lifetime also canbe achieved at higher growth rates and lower cost than were previouslypossible, due to the simplified crystal growing apparatus. The siliconproduced by the process according to this invention has a particularadvantage in providing more efficient, low-cost high lifetime solarcells.

Scope

While the invention has been described in connection with what arepresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not limited to thedisclosed embodiments and alternatives as set forth above, but on thecontrary is intended to cover various modifications and equivalentarrangements included within the scope of the forthcoming claims. Forexample, other materials that are amenable to being grown by the CZprocess may be employed as the melt material, such as gallium arsenide,gallium phosphide, sapphire, and various metals, oxides and nitrides.Also, other materials that are resistant to breakdown by molten silicon,such as ceramic coatings, or various metals, oxides, nitrides, andcombinations thereof can be used for the composition of the crucible, oras a coating on interior walls of the crucible.

Separate weirs or baffles can be provided to protect the crystal fromconvection currents and thermal perturbations. Multiple pulling chamberscan be provided for continuous pulling of successive crystals withoutnecessarily being rotated in place over the crucible. What is importantis that a succession of crystal pulling chambers are sequentiallypositioned over the crucible for repeated, substantially continuouscrystal growth without the need to remove the crucible after each growthcycle. Therefore, persons of ordinary skill in this field are tounderstand that all such equivalent arrangements and modifications areto be included within the scope of the following claims

1. A system for achieving improved level control for continuouslyreplenishing a melt of crystalline feedstock in a crucible forcontinuous CZ crystal growth, wherein an ingot is pulled from arespective seed crystal positioned at a melt/crystal interface in thecrucible comprising: a weight sensor means attached for weighing thecrucible and for converting a load or weight exerted by the crucibleinto output signals representative of that sensed load or weight; acontroller communicatively linked with the weight sensor means forreceiving output signals from the weight sensor representative of theweight of the crucible, for determining the weight of the growthcrucible both empty and with a desired depth of melt at the crystalgrowth interface, and for providing activation signals over a secondcommunication link to a dispenser for selectively providing siliconfeedstock to the crucible, wherein the controller further comprises acomputer means for determining activation of the dispenser based on adesired depth D of melt in the crucible according to the followingrelationship:D=(W−Wt)/(π*R exponent 2*r); where W is the total weight of the cruciblecontaining melt; Wt is the weight of the crucible measured empty; R isthe internal radius of the crucible; and r is the density of liquidsilicon; and a source of silicon feedstock connected to the dispenser,such that the dispenser, in response to the output signals from thecontroller, releases a predetermined amount of solid silicon feedstockinto the crucible to maintain the melt level at the growth/crystalinterface at a desired depth.
 2. A system for achieving improved levelcontrol for continuously replenishing a melt of crystalline feedstock ina crucible for continuous CZ crystal growth as in claim 1, wherein theweight sensor means further comprises one or more load cells, eachhaving an arm, coupled with the crucible, capable of deflection inresponse to an applied load, such that weight of silicon melt in thecrucible produces a deflection that in turn produces an electricalresistance change proportional to the load.
 3. A system for achievingimproved level control for continuously replenishing a melt ofcrystalline feedstock in a crucible for continuous CZ crystal growth asin claim 1, wherein the communication links between the weight sensor,controller and dispenser comprise electrical cable, fiber optic cable,wireless or infra red links to provide stable high temperatureoperation.